The present invention relates to control of moisture inside a packaged electronic device and relates particularly to an improved desiccant and desiccant package which desiccates highly moisture-sensitive electronic devices to prevent premature device failure or premature degradation of device performance.
Various microelectronic devices require humidity levels in a range of about 2500 to below 5000 parts per million (ppm) to prevent premature degradation of device performance within a specified operating and/or storage life of the device. Control of the environment to this range of humidity levels within a packaged device is typically achieved by encapsulating the device or by sealing the device and a desiccant package within a cover. Desiccant packages include a container for receiving solid water absorbing particles (a desiccant) or providing such particles into a binder. Examples of solid water absorbing particles include molecular sieve materials, silica gel materials, and materials commonly referred to as Drierite materials which are used to maintain the humidity level within the above range.
Particular microelectronic devices, for example, organic light-emitting devices (OLED) or panels, polymer light-emitting devices, charge-coupled device (CCD) sensors, and micro-electro-mechanical sensors (MEMS) require humidity control to levels below about 1000 ppm and some require humidity control below even 100 ppm. Such low levels are not achievable with desiccants of silica gel materials and of Drierite materials. Molecular sieve materials can achieve humidity levels below 1000 ppm within an enclosure if dried at a relatively high temperature. However, molecular sieve materials have a relatively low moisture capacity at humidity levels at or below 1000 ppm, and the minimum achievable humidity level of molecular sieve materials is a function of temperature within an enclosure: moisture absorbed, for example, at room temperature, can be released into the enclosure or package during temperature cycling to higher temperature, such, as, for example, to a temperature of 100xc2x0 C. Solid water absorbing particles used within such packaged devices include 0.2 to 200 xcexcm particle size powders of metal oxides, alkaline earth metal oxides, sulfates, metal halides, or perchlorates, i.e. materials having desirably relatively low values of equilibrium minimum humidity and high moisture capacity. However, such materials even when finely divided into powders of 0.2 to 200 xcexcm particle size often chemically absorb moisture relatively slowly compared to the above-mentioned molecular sieve, silica gel, or Drierite materials. Such relatively slow reaction with water vapor leads to a measurable degree of device degradation of performance following the sealing of the desiccant inside a device cover due to, for example, moisture absorbed on the inside of a device, moisture vapor present within the sealed device, and moisture permeating through the seal between the device and the cover from the outside ambient.
Some solid water absorbing particles, particularly molecular sieve materials which entrain moisture by physical absorption within microscopic pores, require a dehydrating step at substantially elevated temperature prior to use within a device enclosure, thus increasing the number of process steps and calling for additional apparatus, such as, for example, a controllable furnace to achieve substantial dehydration.
Selection of solid water absorbing particles and the method of applying selected particles to an inner portion of a device enclosure prior to sealing the device within or by the enclosure is governed by the type of device to be protected from moisture. For example, highly moisture-sensitive organic light-emitting devices or polymer light-emitting devices require the selection of particular solid water absorbing particles and methods of application, since organic materials or organic layers are integral constituents of such devices. The presence of organic materials or layers may, for example, preclude the use of certain solvents or fluids in the application of a solid water absorbing particles dispersed in a fluid to organic-based devices. Furthermore, a thermal treatment, if required, of a desiccant contained within a sealed device enclosure, needs to be tailored to the constraints imposed by thermal properties of the organic constituents or layers of the device. At any rate, release of solvent vapors during a thermal treatment of a desiccant disposed within a sealed device enclosure must be avoided or minimized if solvent vapors can adversely affect organic constituents of organic-based electronic devices. The aforementioned considerations pertaining to organic-based electronic devices may not be as important if the electronic device to be desiccated is strictly an inorganic or metallic device such as, for example, a MEMS device or a CCD sensor without an organic color filter overlay.
For highly moisture sensitive electronic devices, such as organic light-emitting devices or polymer light-emitting devices, VanSlyke, U.S. Pat. No. 5,047,687 teaches the use of a protective layer comprised of a mixture of at least one organic component of the organic electroluminiescent medium and at least one metal having a work function in the range of from 4.0 to 4.5 eV capable of being oxidized in the presence of ambient moisture. The metal in the protective layer is described by VanSlyke as being sufficiently reactive to be oxidized by ambient atmospheric moisture over an extended period of time when incorporated into the organic EL device. In this use the metal is used as solid water absorbing particles for moisture in the protective layer. That neither a coated layer of metal film alone nor successively coated layers of the metal and organic films were effective in preventing the dark spot growth due to ambient moisture was attributed to the slow oxidation of the bulk metal. VanSlyke, therefore, teaches that the oxidation susceptibility of reactive metals that can be oxidized by ambient moisture is enhanced by the higher surface to volume ratios achieved by co-deposition of the metal into a mixed layer of metal and an organic medium. However, VanSlyke does not teach the required metal desiccant particle size for optimal moisture absorption protection nor does he teach the effect of metal particle size on performance in protecting organic EL devices.
Numerous publications describe methods and/or materials for controlling humidity levels within enclosed or encapsulated electronic devices. For example, Kawami et al., European Patent Application EP 0 776 147 A1 disclose an organic EL element enclosed in an airtight container which contains a drying substance comprised of a solid compound for chemically absorbing moisture. The drying substance is spaced from the organic EL element, and the drying substance is consolidated in a predetermined shape by vacuum vapor deposition, sputtering, or spin-coating. Kawami et al. teach the use of the following solid water absorbing particles: alkali metal oxides, alkali earth metal oxides, sulfates, metal halides, and perchlorates. Kawami et al., however, do not teach the effect of particle size of these solid water absorbing particles on their performance.
Shores, U.S. Pat. No. 5,304,419 discloses a moisture and particle getter for enclosures which enclose an electronic device. A portion of an inner surface of the enclosure is coated with a pressure sensitive adhesive containing a solid desiccant with average particle size usually 0.2 to 100 xcexcm and preferably 0.5 to 10 xcexcm.
Shores, U.S. Pat. No. 5,401,536 describes a method of providing a moisture-free enclosure for an electronic device, the enclosure containing a coating or adhesive with desiccant properties. The coating or adhesive comprises a protonated alumina silicate powder with average particle size 0.2 to 100 xcexcm, preferably 1 to 10 xcexcm, dispersed in a polymer.
Shores, U.S. Pat. No. 5,591,379 discloses a moisture gettering composition for hermetic electronic devices. The composition is applied as a coating or adhesive to the interior surface of a device packaging, and the composition comprises a water vapor permeable binder which has dispersed therein a desiccant with average particle size of 0.2-100 xcexcm, preferably 0.3-50 xcexcm, which is preferably a molecular sieve material.
Many of the desiccants disclosed by Shores will not function effectively with highly moisture-sensitive devices at a humidity level lower than 1000 ppm. In addition, Shores does not teach why the particle sizes disclosed are chosen or the effect of particle size on the performance of the desiccants.
Similarly, binders, such as polyethylene disclosed by Shores, that have low moisture absorption rates compared to the absorption rate of the pure selected desiccants would not function effectively to achieve and to maintain a humidity level below 1000 ppm during a projected operational lifetime of a highly moisture-sensitive device.
Deffeyes, U.S. Pat. No. 4,036,360 describes a desiccating material that is useful as a package insert or on the interior walls of packaging boxes for applications requiring only moderate moisture protection, such as film or cameras. The material comprises a desiccant and a resin having a high moisture vapor transmission rate.
The desiccants disclosed by Deffeyes are alumina, bauxite, calcium sulfate, clay, silica gel, and zeolite, but Deffeyes does not describe the particle size of any of the desiccants. None of these desiccants will function effectively with highly moisture-sensitive devices at a humidity level lower than 1000 ppm. In addition the moisture vapor transmission rate requirement for the resin is not adequately defined since there is no reference to the thickness of the measured resins. A material that transmits 40 grams per 24 hrs per 100 in2 at a thickness of 1 mil would be very different than one that transmits 40 grams per 24 hrs per 100 in2 at a thickness of 100 mils. It is therefore not possible to determine if the moisture vapor transmission rates disclosed by Deffeyes are sufficient for highly moisture-sensitive devices.
Taylor, U.S. Pat. No. 4,013,566 describes solid desiccant bodies that are useful as drier materials in refrigerant fluid systems. The solid desiccant body comprises finely divided particles of desiccant material bound in a moisture transmissive aliphatic epoxy polymer matrix.
The desiccants disclosed by Taylor are molecular sieves, activated alumina, and silica gel. Taylor teaches the use of particle sizes 1 to 10 xcexcm, but does not teach the impact of particle size on desiccant performance. None of these desiccants will function effectively with highly moisture-sensitive devices at a humidity level lower than 1000 ppm. In addition the moisture vapor transmission rate requirement for the resin is not adequately defined; stating only that the solid desiccant bodies have rates of adsorption of absorption comparable to the desiccant materials alone. It is therefore not possible to determine if the resins disclosed by Taylor are sufficient for highly moisture-sensitive devices.
Booe, U.S. Pat. No. 4,081,397 describes a composition used for stabilizing the electrical and electronic properties of electrical and electronic devices. The composition comprises alkaline earth oxides in an elastomeric matrix.
The desiccants disclosed by Booe are barium oxide, strontium oxide, and calcium oxide. Booe teaches the use of particle sizes less than 80 mesh (177 xcexcm) to minimize the settling of oxides within the suspension. Booe does not teach the impact of particle size on desiccant performance. These desiccants will function effectively with highly moisture-sensitive devices at humidity levels lower than 1000 ppm; however, Booe claims the elastomeric matrix has the property of retarding the rate of fluid absorption of the alkaline earth particles. In the examples the water absorption rate of the compositions are 5 to 10 times slower than the alkaline earth particles alone. This decrease in absorption rate is disclosed as a desirable feature that improves the handling of the highly reactive alkaline earth oxides. In highly moisture-sensitive devices, however, any decrease in the absorption rate of moisture will increase the likelihood of device degradation, and identification of resins that will increase the absorption rate of moisture would be highly desirable. For highly moisture-sensitive devices, therefore, it is important to determine the minimum allowable water vapor transmission rate of the binders used in combination with effective desiccant materials.
Boroson et al., U.S. Pat. No. 6,226,890 describes a method of desiccating an environment surrounding a moisture-sensitive electronic device sealed within an enclosure, including selecting a desiccant comprised of solid particles having a particle size range 0.1 to 200 micrometers. The desiccant is selected to provide an equilibrium minimum humidity level lower than a humidity level to which the device is sensitive within the sealed enclosure. A binder is chosen that maintains or enhances the moisture absorption rate of the desiccant for blending the selected desiccant therein. The binder may be in liquid phase or dissolved in a liquid. A castable blend is formed including at least the desiccant particles and the binder, the blend having the solid water absorbing particles comprise 10 wt % to 90 wt % of the solid water absorbing particles and the binder.
The blend is cast in a measured amount onto a portion of an interior surface of an enclosure to form a desiccant layer thereover, the enclosure having a sealing flange. The blend is solidified to form a solid desiccant layer, and the electronic device is sealed with the enclosure along the sealing flange. Boroson et al., however, do not teach the effect of particle size of these solid particle desiccants on their performance, nor do they teach any benefit of particles smaller than 0.1 micrometers.
It is an object of the present invention to provide a desiccant package which includes a desiccant for protecting highly moisture-sensitive electronic devices sealed within an enclosure.
It is another object of the present invention to provide a desiccant for protecting highly moisture-sensitive electronic devices sealed within an enclosure. This object is achieved by a desiccant comprising solid water absorbing particles of one or more materials, at least one of such materials having an average particle size range 0.001 to 0.1 micrometers to provide a high rate of water absorption and to provide an equilibrium minimum humidity level lower than a humidity level to which the device is sensitive within the sealed enclosure.
These objects are achieved by a desiccant package useable for protecting highly moisture-sensitive electronic devices sealed within an enclosure, comprising:
a) a moisture-permeable container which can be positioned in the sealed enclosure;
b) solid water absorbing particles of one or more materials disposed in the moisture-permeable container;
c) said solid water absorbing particles including solid particles of one or more materials, at least one of such materials having an average particle size range 0.001 to 0.1 micrometers to provide a high rate of water absorption and to provide an equilibrium minimum humidity level lower than a humidity level to which the device is sensitive within the sealed enclosure; and
d) said moisture-permeable container essentially maintains the moisture absorption rate of the solid water absorbing particles contained therein, the moisture-permeable container acting to separate the solid water absorbing particles from the highly moisture-sensitive device.
The present invention provides the following advantages: a moisture absorption rate that enhances the moisture absorption rate of a solid material capable of providing a low equilibrium minimum humidity within the enclosure, by the reduction of said solid material particle size to an average less than 0.1 micrometers; a moisture absorption rate that essentially maintains or enhances the moisture absorption rate of said less than 0.1 micrometer solid water absorbing particles capable of providing a low equilibrium minimum humidity within the enclosure, by a container or binder in which the solid water absorbing particles are contained; simple, fast, and reliable placement of a desiccant layer on an interior surface of a device enclosure; containment of solid desiccant particles by a container or binder within the device enclosure; thermally curable binders provide for removal of moisture trapped in an uncured desiccant layer by thermal curing of the layer; radiation-curable binders provide for fast curing of a desiccant layer by exposure to radiation; forming a desiccant layer on a separate adhesively bondable support or between a water permeable membrane and a separate adhesively bondable support provides for high speed, roll-to-roll manufacturing of a desiccant layer supply; and providing a desiccant layer having relatively low sensitivity of its desiccation efficacy to temperature cycling at elevated temperature up to 150xc2x0 C.